What Is Hot Isostatic Pressing?

Hot isostatic pressing (HIP) process is to place the product in a closed container, apply equal pressure to the product at the same time, and apply high temperature. Under the action of high temperature and pressure, the product can be sintered and densified. Hot isostatic pressing is an indispensable means for the production of high-performance materials and the development of new materials; hot isostatic pressing can be directly powder-molded, and the powder is packed into a jacket (similar to the role of a mold). The jacket can be made of metal or ceramic (low Carbon steel, Ni, Mo, glass, etc.), and then use nitrogen or argon as the pressurizing medium to make the powder directly heated and pressurized and sintered to form a powder metallurgy process; or the formed castings; including aluminum alloys; titanium alloys; high temperature Castings with shrinkage holes such as alloys are subjected to thermal densification. After hot isostatic pressing, the castings can be 100% densified, improving the overall mechanical properties of the castings.

The first HIP device with a hot-wall threaded structure was successfully developed by the American Battelle Institute in 1955. The equipment has an inner diameter of only 4.76mm, a pressure of 13.78MPa, and a temperature of 816 ° C. In 1956, a cold-wall HIP device appeared. At the beginning of the 1960s, the Swedish ASIA company successfully developed a prestressed steel wire (belt) winding structure frame HIP device. After that, the company produced large-scale HIP devices with a volume of 1290mm × 2500mm, a temperature of 1450 ° C, and a pressure of 320MPa. Since the 1980s, the trend of simply expanding the size of HIP devices has declined, and it has developed in the direction of single functions and special applications. For example, ultra-high temperature (up to 3000 ° C), ultra-high pressure (up to 980MPa), fast, impregnated, and HIP devices using multiple gases have appeared one after another. In order to meet the requirements of liquid phase sintering (such as sintering hard alloys), low-pressure hot isostatic pressing devices (also known as overpressure sintering furnaces, sintering-hot isostatic presses, etc.) have been developed with a pressure of only 9.8 to 30 MPa. This device completes wax removal, pre-firing and sintering in the production of cemented carbide in one process. Three hot wall HIP devices were manufactured in China in the 1960s; cold wall HIP devices began to be manufactured in 1972, were put into use in 1975, and the first steel wire winding HIP device was built in 1979. ^[1]

The hot isostatic pressing device is mainly composed of a water-cooled pressure vessel, a heating furnace which is insulated from the pressure vessel, and additional facilities such as control, safety and operation. Pressure vessels are sealed in two ways: threaded and frame. Thread seals can only be opened, closed and loaded from the top. The frame seal can be opened and closed simultaneously from the top and bottom as shown in the figure. Both pressure vessels and frames can be wound with steel wires (belts) to withstand high pressure. Generally, argon is used as the pressure transmitting medium, and its thermal conductivity is lower than that of helium. It is easy to make the temperature zone reach the required temperature and make the temperature zone temperature uniform. At the same time argon is cheaper than helium. Due to the convection of the compressed gas between the cold wall of the pressure vessel and the heating furnace, the temperature distribution of the compact is not uniform, which is the difficulty of HIP. An appropriate heat insulation layer can be provided between the heating element and the cold wall, and between the heating element and the workpiece to separate the pressing space, thereby overcoming this difficulty. ^[2]

The basic operation steps of HIP are: put the powder or powder compact into the package; remove the gas adsorbed on the powder surface, the space between the powder and the package; seal the package under vacuum and place it in a pressure vessel with a heating furnace After the pressure vessel is sealed, an inert gas (ie, a pressure transmitting medium) is pumped to a certain pressure; then the temperature is raised to the required temperature, and due to the expansion of the gas volume, the pressure in the vessel also rises to the required pressure. The forming and sintering are completed under the action of high temperature and high pressure. Thereafter, the envelope is removed by mechanical or acid dipping to obtain a product. Metal powder covers are usually welded from steel plates that have undergone strict leak detection. Any slight leakage will cause thermally induced porosity in the product. The cladding material can also be used for glass or ceramics with uniform viscous flow at the sintering temperature. For example, quartz has the necessary viscosity at high temperatures and is suitable for pressing superalloys. Glass envelopes can be made by slurry forming and sintering methods.

In engine manufacturing, hot isostatic presses have been used to form powder superalloy turbine discs and gas discs. The high-temperature alloy powder is filled into a vacuum-formed thin-walled forming envelope, and hot isostatic pressing is performed after welding. After removing the envelope, a compact disk close to a desired shape can be obtained. Powder hot isostatic pressing materials generally have a uniform fine grain structure, which can avoid macro segregation of the ingot and improve the process and mechanical properties of the material. Powder superalloy hot isostatic pressing or hot isostatic pressing and forging of disk parts have been applied to a variety of high thrust-to-weight ratio aircraft engines. Similarly, hot isostatic pressing is also used to make powder titanium alloy fan disks and powder aluminum alloy and powder titanium alloy load-bearing components on aircraft. In the spacecraft manufacturing industry, hot isostatic pressing is mainly used to make dense carbonaceous structural parts, such as the rudder surface of a rocket and the throat liner of a solid rocket engine.

Precision castings of various alloys, such as high-temperature alloy turbine blades, cast titanium casings, and aluminum alloy castings of turbochargers, etc., can reduce internal porosity and shrinkage, and improve performance, reliability and Service life. Hot isostatic pressing is also an effective way to repair old parts to extend their useful life.

The advantage of hot isostatic pressing technology is that it combines the advantages of hot pressing and isostatic pressing. It has a low forming temperature, compact products, and excellent performance, so it is a necessary means for the preparation of high-performance materials. At present, it has been implemented in the United States, Japan, and Europe. Industrialization, application in marine, aviation, aerospace, automotive and other fields; China started late, in the 1960s, some domestic scientific research units carried out a series of research, including the development of hot isostatic pressing equipment, aviation high-temperature alloy powder The development of turbine disks and the research and development of hot isostatic high-performance materials have won a series of research results and scientific and technological progress awards. The industrialization of hot isostatic pressing started late in China. At present, the popularity of this technology in China is extremely high. Low, because there is no scale and industrialization, so the production cost is relatively high. At present, the technology is mainly concentrated on the research and development of aerospace high-performance materials and the compaction of castings; but with the development of the economy, the demand for building innovative countries, As well as the development of some high-performance materials, some domestic units rely on the research results of scientific research institutes to carry out the hot isostatic pressing industrialization. Work; made fruitful work for the development of new materials in China and the popularization of hot isostatic pressing.